The field of the invention is systems and methods for magnetic resonance imaging (“MRI”). More particularly, the invention relates to systems and methods for reconstructing spatial and spectral imaging data, for example, when employing hyperpolarized imaging techniques.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the excited nuclei in the tissue attempt to align with this polarizing field, but process about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited nuclei or “spins”, after the excitation signal B1 is terminated, and this signal may be received and processed to form an image.
When utilizing these “MR” signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Magnetic resonance spectroscopy (“MRS”) may be used in vivo for the determination of individual chemical compounds located within a volume of interest. The underlying principle of MRS is that atomic nuclei are surrounded by a cloud of electrons that slightly shield the nucleus from any external magnetic field. As the structure of the electron cloud is specific to an individual molecule or compound, the magnitude of this screening effect is then also a characteristic of the chemical environment of individual nuclei. Since the resonant frequency of the nuclei is proportional to the magnetic field it experiences, the resonant frequency can be determined not only by the external applied field, but also by the small field shift generated by the electron cloud. Detection of this chemical shift, which is usually expressed as “parts per million” (“ppm”) of the main frequency, requires high levels of homogeneity of the main magnetic field, B0.
Typically, MR proton spectroscopy is used to generate a one-dimensional (1D) frequency spectrum representing the presence of certain chemical bonds in the region of interest. In medical diagnosis and treatment, MRS provides a non-invasive means of identifying and quantifying metabolites from a region of interest, often the human brain. By finding the relative spectral amplitudes resulting from frequency components of different molecules, medical professionals can identify chemical species and metabolites indicative of diseases, disorders, and other pathologies such as Alzheimer's disease, cancer, stroke, and the like. In this context, two nuclei are typically of particular interest, hydrogen-1 and phosphorous-31. Phosphorus 31 MRS is directed to the detection of compounds involved in energy metabolism relating to membrane synthesis and degradation. Metabolites of particular interest in proton MRS studies include glutamate (Glu), glutainine (Gln), choline (Cho), creatine (Cre), N-acetylasparate (NAA), and the inositols (ml and sl).
With newer contrast agents such as hyperpolarized 13C, metabolic processes can be observed in the human body, such as in the context of cancer detection, by analyzing the signal contributions from various metabolites in regions of interest. MR imaging of 13C holds the potential to probe pathology at a molecular level. Unfortunately, signal from endogenous 13C is substantially indistinguishable from noise due to low receptivity, sub millimolar in-vivo concentrations, and scan-time limitations.
Recent advances in hyperpolarization now allow for 10,000 fold increases in polarization, enabling the use of 13C labeled molecules as tracers. With modern dynamic nuclear polarization (DNP), many important endogenous, organic molecules can be polarized and subsequently imaged. For example, hyperpolarized (HP) [1-13C]pyruvate shows potential as a molecular biomarker of cellular metabolism. In experiments, a small sample of [1-13C]pyruvate has been hyperpolarized and intravenously injected. In-vivo, it is transported into the cell and undergoes rapid conversion into its primary metabolic byproducts, [1-13C]lactate, [1-13C]alanine, and 13C bicarbonate.
Imaging of HP 13C compounds is more challenging than many other contrast agents because the transient increase in polarization decays at a rate governed by the nuclear spin-lattice relaxation time, T1. Also, once injected and subsequently imaged in-vivo additional decay is observed due to radio frequency (RF) excitation, and chemical and spin exchange with the endogenous environment. As such, with each passing second following removal from the polarizer, the HP 13C compounds become less effective contrast agents.
Furthermore, spectral imaging with hyperpolarized species presents substantially different challenges than with endogenous, thermally polarized nuclei. While the signal-to-noise ratio (SNR) is quite high for a single RF excitation, each excitation consumes a portion of the finite hyperpolarization. Due to perfusion and washout, 13C molecules are imaged while undergoing multiple dynamic processes. Thus, dynamic imaging is essential for potential quantification of apparent conversion rates. Conventional spectroscopic imaging techniques, such as chemical shift imaging (CSI), provide excellent spectral resolution, but the long imaging time (˜10 s) and the need to phase encode both spatial dimensions limits the ability to measure dynamics. Rapid spectroscopic imaging techniques, such as echo planar spectroscopic imaging (EPSI) and spiral CSI, greatly reduce the scan time and RF deposition but are often limited by a narrow spectral bandwidth, a consequence of gradient slew-rate limitations and the reduced 13C gyromagnetic ratio (˜4× lower than 1H).